This invention relates to the treatment of lithographic printing plates, and more particularly to the treatment thereof with an organic titanate solution to thereby strengthen resin image areas and increase the solvent resistance thereof.
Lithographic printing plates typically consist of oleophilic or ink receptive image areas and hydrophilic background areas. As was discussed in U.S. Pat. No. 3,136,637, the oleophilic image areas are subject to abrasion during the printing process and to the effects of harsh solvent treatments, often resulting in a reduction in ultimate press life of such plates. Furthermore, lithographic aluminum surfaces, which conventionally provide the background areas of the plate, are known to be easily oxidized. As such oxidation occurs the hydrophilic surface can tend to become somewhat oleophilic in nature and when printing ink contacts this surface, toning or scumming results. Such a condition necessitates the use of other chemicals, such as acids, to restore the original hydrophilic nature of the substrate. Further, exposure of the aluminum surface to elevated temperatures, such as during post treatment of the plate, often accelerates the tendency towards oxidation and thus toning.
Electrostatically imaged printing plates, whose toner powder image areas are typically composed of a finely divided particulate mixture of resin, carbon black and magnetite, require heat treatment to fix the powder image to the substrate. In the case of aluminum substrates, the heat fixing can adversely effect the background areas to again increase the tendency towards toning and correspondingly reduce press life. A method of allowing heat fixing while allaying the problem of toning has long been sought.
The aforementioned plates also react adversely to certain types of press-wash, solvents and ultraviolet-cured printing inks. These inks and the solvents recommended for use with them can reduce the adhesion of conventional resin images to the substrate and/or weaken the internal strength of the image-forming resin. In either case the predicted useful life of the plate on the printing press can be severely limited. Also, dampening solutions, and particularly those containing isopropyl alcohol, have a tendency to lessen the press life of the printing plate by slightly solvating the resin-capped images.
One common method for increasing the resistance of the image areas to solvents and the strength of the image area is by utilizing crosslinkable photopolymer systems as the image forming material on the lithographic plate. This then provides tough, durable image areas. However, electrostatic images and those formed from light sensitive systems which do not crosslink upon light exposure, e.g. conventional diazo resins, can be definitely enhanced by treatment as hereinafter described. Furthermore, the hydrophilic background areas are effectively preseved by the described treatment and their tendency to oxidize is reduced.